1. Field of the Invention
The invention pertains to the field of passive fiber optic sensors and more particularly to passive optical sensing devices for the measurement of physical parameters such as displacement, force, or pressure where the sensitive element is coupled by optical fibers from remote locations to the processing units.
2. Description of the Prior Art
There are a wide variety of passive optical sensors the modulate light intensity as a function of a physical parameter to be measured. In many applications it is desirable to locate the optical sensor at a remote location and couple it to a light source and data processing equipment via optical fibers. Sensors of this type are particularly desirable in aircraft systems wherein the light weight and freedom from electro-magnetic interference of the optical fibers are of prime importance. A requirement of these optical systems is that the sensor data not be distorted by transmission power loss in the optical fibers. Additionally, it is required that a recalibration of the system not be necessary when the passive optical sensor element, the light source, or the data receiver are replaced during normal aircraft maintenance.
In many prior art systems utilizing optical sensors, sensitivity and parameter response area primary concern and absolute value calibration is not a consideration, while in other prior art applications absolute value calibration is provided. In systems providing an absolute calibration the device is calibrated after the sensor and the optical fibers have been installed. Still other systems employing analog optical sensors utilize wavelength multiplexing of two optical signals transmitted over the same optical fiber to eliminate sensitivity to transmission and coupling losses. Such systems provide two signals at different frequencies with the ratio of the intensity of one to the intensity of the other forming the measured parameter. This intensity ratio is modulated in accordance with the physical parameter to be measured. Such multiplexing systems are complex and require two sources, one for each of the two frequencies and two detectors or filtered detector combinations. Variations in source and detector gains result in a change of signal ration and hence establish a calibration error.
A time multiplex system in the prior art is disclosed in U.S. Pat. No. 4,432,599. In this system light from a single source is coupled from a common optical fiber to two optical fibers of equal length by a directional couplers, separated by a predetermined distance along the common fiber, to be incident to a mirror attached to a physical parameter sensor and moveable in accordance with the measured parameter. The relative intensity of the light reflected to the fibers is a function of the mirror position and consequently the value of the physical parameter. Calibration of this system is dependent upon the relative attenuation of the two fibers guiding light between the common fiber and the mirror. Variations in the relative attenuation of the fibers, which may be caused by crimping one or the other during normal maintenance, seriously affects the calibration and the accuracy of the overall detection system. Additionally, the operation of the system is depending upon maintaining equal lengths for the branching optical fibers, any stretching of one relative to the other affects the time demultiplexing and may render the entire system inoperable.